Production of pure silicon



digestion, using; hydrofluoric acid.

Unit Sim ace wee PRODUCTION OF PURE-SILICON; 5 Walter Voos, Gampel, Switzerland, assignor to Loiiza Electric and Chemical Works Limited, Basel, Switzerl nd, a co pa q swi zerlandi' silows:

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' The individual operating steps are performed as fol The leaching of'the metal powder is done with dilute hydrochloric acid, dilute sulfuric-acid, or av mixture of these two acids I prefer to'use dilute hydrochloric acid.

' mswh n i i i m wi ze t. n. 12.1 5 1 not only hydrogen but also; self-igniting silicjon hydrides which may cause}dangerousexplosionswhen; the work is done on a commercial scale; Various proeessesflhave" been; developed and suggestedito overcome this-grave. drawback, but all the suggested processes are complicated and expensive and increase the eostoff thev product substantially.

There has not been, any commercial process so far which-would permitthe production ofsilicon' ofa' very high purity in a plant having low first costs and1l owopcrating costs. y 1

My present invention, aims at silicon with. a silicon content of not less than-99.5"percentffor instance, 919 197 percent, and as high as 99.99 percent. I f

The primary object-of my invention is to} provide a simple and inexpensive process that: will obviate the possibility of explosions; otherwise existent, even when large quantities of material are handled and will thus be particularly suitable for commercial processing on a large scale.

An additional object'of the' invention is to provide a hydrometallnrg-ieal digesting process. 7

Another object of this invention is to sta rt' from crude silicon with a content of approximatelyfid to; 98% Si as-araw material: The crudemetafisobtained inmetal? lnrgical plants.

Further objects of this-invention will become apparent from, the.following descriptionc To. prepare. the crude. metal for. the digestion, it is-first cornminuted to a particle size of approximately 3 mm.

andthen pulverized to pass:through a screen with a mesh aperture of 0.075 mm. 7 I

The process of the invention consists basically in two successive sections of treatments, namelyj,

('1) An acid digestion-and. (2) Leaching in heapsif The concentration of the dilute acid should be such as to be capable'ofextracting all secondary constituent metals, such asFe, Al, Ca, Mg, Mn, Ti, as far as they are atall soluble in dilute hydrochloric acid, dilute sulfuric acid, or mixtures thereof.

, of 1:1. I

When dilute hydrochloric ac1d is used, I prefer a concentration of 200 g./l.. HCl. Such concentration may readily be prepared by diluting. a concentrated acid having an HClcontent of 400 g./l. with water at a ratio The way of adding the dilute acid is of importance. The entire amount; of acid must not be added at once since self-igniting silicon hydri'desgenerate simultaneously with hydrogen, which hydrides, however, are, ashas already been stated, the cause of dangerous explosions.

The greatest care is; necessary in this respect when pure silicon istobeprepared on a large-scale. Accordingjto the invention, the dilute. acid; finely dispersed; for example, in droplets-or in fine streams, is sprayed'on the finely pulverizedcrude silicon metal while the crude metal is actively, mixed at thesame time, for" example, by stirring, or turning over, or simultaneous} stirring and turning over. j By'mixin'g in this way, crude silicon which has beeli freshly sprayed with dilute acid-will immediately be cov-1 ered by other material-and turned over. 7 V

To promote intimate distribution ofthe dilute acid throughout. the crude silicon that is-to-be wetted; it may be convenient to add the dilute acid in several portions." The explosion-danger may be avoided by addingsmaller amounts at intervals or by the addition of-finelyydis persed acid, for instance, in form of droplets or fine streams. I

When the dilute acid is rninutely dispersed, for instance, as a fog of liquid=droplets, while being added, and'the crude silicon metal is at the same time strongly mixed, the dilute. acid may be added continuously. In this manner, the crudemetal' is gradually wetted and saturated. V v

The extraction of metal impurities in the fi'rst section of'operating steps causes. a concentration of'thesilicon "In the firstrsection, the pulverized-crude silicon metal turned over. During this procedure, a' pasty massis formed gradually.

j'IInthe second section,v thepasty mass issubjectedi to heap-leaching for at; least. aday, that is, the mass is left to stand in heaps. without" any me chanical treatment. The, pure highegradesilicon is then separated by wash- .n. a. .....m

content: I I Y i It-is essential fto add only enoaghdilnt'e acid to form a mass. of dough-like consistency. Larger amounts of liquid, Whichwouldform a suspension, are'tobe avoided since this would interfere with the subsequent heapleaching. Relative" to the crude metal, the siliconeontent of' which is generally between 96 and 98%, approximately 30-70%, and preferably Sli -60%, dilute hydro-f chloric acid, or corresponding amounts of sulfuric acidor of a mixture of both acids are required. This means that the digestion of 1,000kg. crude metal will require,- for example, 6001. dilute hydrochloric acid with an H'Glf content of: 1 20 kg. I prefer to add at first only 8G%-'ofthese 6 00 l'. to. each 1,000 kg. of crude-metal while 'mix-' ingvigorously. After this, themass, so to speak, has become immune, and explosions are no longer to be feared. The remaining= 20%' or- '1. of'the dilute acid may be added, without special" precautionary measures, in-stronger continuous streams while-the mass is-- mixed;

The doughlike mass, in the second section, is sub jected'to heap-leaching after the entire amount-of dilute acid hasbeen stirred in. The heaps are left to-rest without" stirring or any. other-mechanical treatment. Hea'p leaching generally requires a rest of at least hours'i L. prefer a resting/.- period of; approximately three-i days.

Patented Feb. 21, 19 61 An extension of the freedom from any movement substantially beyond this limit generally offers no advantages.

After the heap-leaching with dilute acid has been concluded, the pure silicon is obtainedin a known anner by washing and drying. Q

The process as described hereinbefore is suitable for refiningcrude silicon" which contains'only small. amounts of not readily extractablesilicides, such as iron silicide (like FeSi and Fesig). It permits the preparation of pure silicon with Si-contents of 99.5 to 99.7%.

When silicon of ahigher degree of purity, for example, of over 99.7% up to 99.994%, is to be produced, an additional treatment of the dilute acid-digested material with hydrofluoric acid is necessary. The latter is capable of dissolving or decomposing iron silicides, such as FeSi For combined leaching operations withdilu'te hydrochloric acid, dilute sulfuric acid, or mixtures thereof, on the one hand, and with hydrofluoric acid, on the other hand, the material which has been formed withdilute acid into a dough, as has been described above, is immediately mixed with hydrofluoric acid under continuous agitation.

The concentration of the hydrofluoric acid depends on the desired degree of purity and the amount of silicides to be decomposed. Hydrofluoric acid with an HF content up to 30% may be used in this combined leaching operation. For instance, a doughlike mass obtained from 1,000 kg. crude silicon metal powder and 600 l. dilute acid, such as hydrochloric acid diluted with water at a ratio of 1:1, may be treated for further purification, immediately after the formation of the dough, with 392 l. of a hydrofluoric acid solution containing '326 g./l. HP. The HF-content of the liquid phase is, therefore, 127.8 g./l. While the hydrofluoric acid is added, the mass is continuously stirred, and after this addition, the mass is subjected to leaching in, heaps, for example, for three days.

If the purity of the material is to satisfy panticularly high requirements and a metal of a purity in excess of 99.99 percent is to be prepared, the combined leaching operations and the subsequent heap-leaching have to be repeated once or several times. Since the danger of explosions has been eliminated by the first treatment with dilute acid, the repeated acid treatments may be carried out with combined liquors. For example, a mixture of hydrochloric and hydrofluoric acid may be used'in the repeated acid treatment steps.

The acid treatment may he performed in various manners, for instance, by leaching with agitation in open containers, or centrifugal leaching, for example, in a Pachuca tank suitably lined with acid-resistant material, or by pressure-leaching in an autoclave. The heap-leaching is preferably done in the same container by stopping the agitating device.

It will be understood that it is essential to carry out the acid treatment under the above described conditions and to combine the acid treatment with subsequent heap: leaching.

After the heap-leaching, the mass is washed in a known manner and separated from the acid liquor, that is, deacidified, for example, by selective washing, centrifuging, settling and decanting. These operations have to be performed most carefully. The washing operation is not simple because a metal skeleton, that is, a metallic mass of spongy metal grains permeated by fine pores, is formed by the acid digestion and the heap-leaching. The metal grains are diflicult to wash out since they do not readily yield the impurities. The separated liquors may be re-' used after suitable purification. The drying of the purified silicon powder is done, for example, in centrifugal driers or in rotating drum driers at temperatures of 70- 80 C., and preferably at 72-73? C., in a non-oxidizing atmosphere.

. 4v silicon, it can be decarburized before the acid treatment, for example, by heat-treatment, in the absence of air, together with metals or metal compounds which combine with carbon. pi

The following example'smnd tables are given to illustrate the presenrinvenuon witheuginewever; limiting its SCOPE: 1 v L I r EXAMPLE 10,000 kg. crude silicon containing, 98.947% Si'and pulverized to'pass througha 0.075 mm. mesh screen are placed in a container equipped with a mixing device. The metal powder which lies on the bottom of the container is continuously and intensely plowed and turned over.v While this agitation continues, approximately80% (4,800 l.) of the total amount of the acid required for thedigestion (6,000 l. of a dilute hydrochloric acid prepared by mixing equal parts of concentrated hydrochloric 7 acid and water) are slowly stirred in. The acid is added If. it is desired 'to refine a carbide-containingcrude at intervals from a spray. nozzle or in drops whereby the metal powder is gradually formed into a dough-like mass. The overturning is performed so that material freshly sprayed with the dilute acid will immediately be covered by turned-over material and will be included into the turning mass. formation of a pasty mass and safely prevents explosions. It should be noted that particular caution is required at the start of the mixing operation. Once the metal powder is uniformly wetted with the added acid, the grain is immune. The remainder of the dilute hydrochloric acid (approximately 20% of the total or 1,200 1.) may then be added in an uninterrupted stream while the mass is agitated and turned over. There is no further danger of explosions.

Next, heap-leaching follows whereby the material treated with the dilute hydrochloric acid is permitted to rest in the container for three days without any further treatment.

The resulting metal powder paste is now separated from. the liquor by thorough and repeated washing and subsequent decanting and then dried in a rotating drum drier at approximately 72-73 C.

About 9,800 kg. of 99.501% Si are obtained.

All percentage data in the description and in the claims are-to be understood as percent by weight.

The results of several additional digestion runs are shown in the tables below.

- Where the tables refer to g. HCl or g. HF, they indicate weight of pure hydrogen chloride or hydrogen fluoride contained in the respective acids.

Table I [Digestion with dilute hydrochloric sold only, followed by heap-leaching] Table 2 [Digestion with dilute hydrochloric acid, leaching with the addition of dilute hydrofluoric acid (that is, HOl+HF)', and subsequent heapleaching for three days] e g. HOl+g. HF per kg. Metal Finished Product, Percent Bl Raw Material, Percent 8i Thismakes for a gradual and uniform Table 3 [Same treatment as for Table 2, but digestion with HCl-i-HF and heap leaching is repeated twice] (1) Crude silicon metal (2) Grinding to fine powder (3) Digestion by finely dispersed dilute acid (P101 or H2804 or both) and mixing of mass to form a paste (4) Combined leaching with dilute hydrochloric and/or sulfuric acid and hydrofluoric acid (5) Heap-leaching by allowing the paste to rest without mechanical agitation for at least one day and preferably three days (6) Single or multiple repetition of acid trvatment and subsequent heap-leaching (7) Washing and drying l Silicon over 99.7 and up to Silicon 99.5 to 99.7%

It is believed that the general method and the specific examples of my invention, as well as the advantages thereof, will be apparent from the foregoing description. It will also be apparent that while I have shown and described preferred examples of my invention, changes may be made without departing from the spirit of the inven tion, as sought to be defined in the following claims.

I claim:

1. Process for the production on a commercial scale of pure silicon with a content of at least 99.5% Si from crude silicon having a silicon content in the order of magnitude of 96 to 98%, comprising pulverizing the crude silicon so that it will pass through a screen of 0.075 mm. opening, applying acid to the pulverized silicon over a prolonged period of time and in no larger quantity than is needed for the resultant mixture of the pulverized silicon and acid to be of a doughlike consistency, the acid being selected from a group consisting of. dilute hydrochloric acid, dilute sulfuric acid, and a mixture of said acids, vigorously working the pulverized silicon while the acid is added, thereby covering portions of the silicon, which have been freshly treated with acid, with such other portions as have not recently been contacted by acid, further turning over the treated silicon, and forming a doughy mass, subjecting the dough to heap-leaching by allowing the dough to stand in heaps without being moved for at least one day, and isolating the pure silicon by washing and drying.

2. The process as set forth in claim 1, wherein the dilute acid is intermittently added in portions.

3. The process as set forth in claim 1, wherein the dilute acid is continuously added as a fine spray.

4. The process as set forth in claim 1, wherein not more than substantially 80% of the total amount of the dilute acid to be used are added in finely dispersed form, and the remainder of the acid is added rapidly, with the pulverized crude metal being continuously mixed.

5. In the process as set forth in claim 1, the steps of adding hydrofluoric acid to the pasty mass after complete addition of the dilute acid and before heap-leaching, and continuously agitating the pasty mass while sub.- jecting same to the hydrofluoric leaching action.

6. In the process as set forth in claim 5, repeating at least twice the cycle of the first named leaching, the second named leaching, and the heap-leaching.

7. In the process as set forth in claim 6, simultaneously carrying out the first and second named leachings with a mixture of said dilute acid and hydrofluoric acid.

8. The process according to claim 1, wherein the acid is added in an amount of approximately 30 to calculated as dilute hydrochloric acid.

9. In the process as set forth in claim 1, the steps of adding hydrofluoric acid to the pasty mass after the leaching with the dilute acid and before heap-leaching, and continuously agitating the pasty mass while subjecting same to the hydrofluoric leaching action.

10. in the process as set forth in claim 1, said acid being applied in a concentration approximating that of 200 g./l. HCl.

References Cited in the file of this patent UNITED STATES PATENTS 1.386.227 Becket Aug. 2, 1921 1,928,728 Keyes Oct. 3, 1933 FOREIGN PATENTS 488,244 Great Britain July 4, 1938 

1. A PROCESS FOR THE PRODUCTION ON A COMMERCIAL SCALE OF PURE SILICON WITH A CONTENT OF AT LEAST 99.5% SI FROM CRUDE SILICON HAVING A SILICON CONTENT IN THE ORDER OF MAGNITUDE OF 96 TO 98%, COMPRISING PULVERIZING THE CRUDE SILICON SO THAT IT WILL PASS THROUGH A SCREEN OF 0.075 MM. OPENING, APPLYING ACID TO THE PULVERIZED SILICON OVER A PROLONGED PERIOD OF TIME AND IN NO LARGER QUANTITY THAN IS NEEDED FOR THE RESULTANT MIXTURE OF THE PULVERIZED SILICON AND ACID TO BE OF A DOUGHLIKE CONSISTENCY, THE ACID BEING SELECTED FROM A GROUP CONSISTING OF DILUTE HYDROCHLORIC ACID, DILUTE SULFURIC ACID, AND A MIXTURE OF SAID ACIDS, VIGOROUSLY WORKING THE PULVERIZED SILICON WHILE THE ACID IS ADDED, THEREBY COVERING PORTIONS OF THE SILICON, WHICH HAVE BEEN FRESHLY TREATED WITH ACID, WITH SUCH OTHER PORTIONS AS HAVE NOT RECENTLY BEEN CONTACTED BY ACID, FURTHER TURNING OVER THE TREATED SILICON, AND FORMING A DOUGHY MASS, SUBJECTING THE DOUGH TO HEAP-LEACHING BY ALLOWING THE DOUGH TO STAND IN HEAPS WITHOUT BEING MOVED FOR AT LEAST ONE DAY, AND ISOLATING THE PURE SILICON BY WASHING AND DRYING. 